The demand for higher performance steels with optimal combination of properties is becoming more crucial. Since the grain size in steel controls the resulting properties, the desired property profile can be obtained by development of a properly adjusted microstructure.
As-cast steels are prime examples of materials where the properties achieved depend upon the characteristics of the solidification microstructure. In general, a coarse columnar grain structure will inevitably evolve upon solidification if potent heterogeneous nucleation sites ahead of the solidifying front are absent. In the presence of effective seed crystals, fine equiaxed grains form directly in the melt. Depending upon the circumstances, the equiaxed grain structure may completely override the inherent columnar grain formation, which, in turn, gives rise to an improved castability (e.g. hot ductility and hot cracking resistance) through a smaller grain size and reduced problems with centre-line segregation.
Experience has shown that the as-cast microstructures of high alloyed steels are quite different from those of the pure carbon manganese or low alloy steels due to their higher alloy content and broader span in chemical composition. Four distinct solidification modes are commonly observed:    Primary ferrite formation    Primary ferrite formation followed by a peritectic transformation to austenite    Primary ferrite and austenite formation    Primary austenite formation
Due to the absence of subsequent solid state phase transformations, there is particularly a need of grain refining in fully austenitic or ferritic steels. At present, no grain refiners are commercially available for steels, as opposed to cast iron and aluminum alloys where such remedies are widely used to refine the solidification microstructure.
Over the past decades, significant improvement of steel properties has been achieved through strict control of the chemical composition, volume fraction and size distribution of non-metallic inclusions. This has been made possible by the introduction of secondary steelmaking as an integrated step in the production route and the use of advanced ladle refining techniques for deoxidation and desulphurisation. The detrimental effect of inclusions on steel properties arises from their ability to act as initiation sites for microvoids and cleavage cracks during service. Hence, the use of clean steels is normally considered to be an advantage, both from a toughness and a fatigue point of view.
More recently, the beneficial effect of inclusions on the solid state transformation behaviour of steels has been highlighted and recognised. In particular, the phenomenon of intragranular nucleation of acicular ferrite at inclusions is well-documented in low alloy steel weld metals, where the best properties are achieved at elevated oxygen and sulphur levels owning to the development of a more fine-grained microstructure. The same observations have also been made in wrought steel products deoxidised with titanium, although the conditions existing in steelmaking are more challenging due to the risk of inclusion coarsening and entrapment of large particles that can act as initiation sites for cleavage cracks. Because of the problems related to control of the inclusion size distribution during deoxidation and casting, the concept of inclusion-stimulated ferrite nucleation have not yet found a wide application, but is currently limited to certain wrought steel products where the weldability is of particular concern.
Inclusions are known to play an important role in development of the steel solidification microstructure and substantial grain refining has been observed in a number of systems, including
Aluminum-titanium deoxidised low alloy steels due to nucleation of delta ferrite at titanium oxide/nitride inclusions.
Aluminum-titanium deoxidised ferritic stainless steels due to nucleation of delta ferrite at titanium oxide/nitride containing inclusions.
Rare earth metal (REM) treated low alloy steels due to nucleation of delta ferrite at Ce/La containing oxides and sulphides.
Rare earth metal (REM) treated ferritic stainless steels due to nucleation of delta ferrite at Ce/La containing oxides and sulphides.
Rare earth metal (REM) treated austenitic stainless steels due to nucleation of austenite at Ce/La containing oxides and sulphides.
In all cases the grain refining effect is related to the ability of the inclusions to act as efficient heterogeneous nucleation sites, e.g. by providing a low lattice disregistry between the substrate and the nucleus. Experiments have shown that the undercooling required to trigger a nucleation event is of the order of 1° C. when the atomic misfit across the interface is 5% or lower. This degree of undercooling is sufficiently low to promote the formation of an equiaxed microstructure during solidification, provided that number density of the nucleating inclusions ahead of the advancing solid/liquid interface exceeds a certain threshold.
FeSi-based inoculants and treatment alloys for cast iron are commercially available and commonly used in the foundry industry. These alloys contain balanced additions of strong oxide and sulphide formers such as Ca, Al, Ce, La, Ba, Sr or Mg. It is well established that the major role of the minor elements is to modify the chemical composition and crystal structure of the existing inclusions in the liquid iron, thus promoting the graphite formation during solidification. This occurs by a process of heterogeneous nucleation analogous to that documented for grain nucleation in steel.
Experiments have shown that both low carbon (LC) FeCr and FeMn, produced by means of conventional casting methods, contain an intrinsic distribution of oxides and sulphides, the former group being most important. These systems have a high oxygen solubility in the liquid state (about 0.5% O by weight or higher), where the inclusions form naturally both prior to and during the casting operation owing to reactions between O and S and Cr, Si and Mn contained in the alloys. However, because the cooling rate associated with conventional sand mould casting is low, the resulting size distribution of the Cr2O3, SiO2, MnO or MnS oxide and sulphide inclusions is rather coarse. Typically, the size of the inclusions in commercial LC FeCr and FeMn is between 10 and 50 μm, which make such alloys unsuitable for grain refining of steel.
Controlled laboratory experiments have shown that the additions of a strong oxide and sulphide former such as Ce to a liquid ferrous alloy will result in the formation of Ce2O3 and CeS. These inclusions are similar to those observed in steels treated with rare earth metals, and in both cases extensive grain refinement is achieved. The initial size of the inclusions obtained with this conventional alloying technique is between 1 and 4 μm. However coarsening of the inclusion population occurs gradually with time after the Ce addition, and unless the melt is rapidly quenched thereafter the inclusions will grow large and eventually become detrimental to mechanical properties. Thus, the real challenge is either to create or introduce small non-metallic inclusions in the liquid steel that can act as heterogeneous nucleation sites for different types of microstructures during solidification and in the solid state (e.g. ferrite or austenite), without compromising the resulting ductility or fracture toughness. In practice, this can be achieved by the use of a novel alloying technique, based on additions of tailor made grain refining alloys to liquid steel where the necessary reactants or seed crystals are embedded.